# std::experimental::pmr::polymorphic_allocator<T>::construct

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< cpp‎ | experimental‎ | polymorphic allocator

template< class U, class... Args > void construct( U* p, Args&&... args ); | (1) | (library fundamentals TS)
template< class T1, class T2, class... Args1, class... Args2 > void construct( std::pair<T1, T2>* p, std::piecewise_construct_t, std::tuple<Args1...> x, std::tuple<Args2...> y ); | (2) | (library fundamentals TS)
template< class T1, class T2 > void construct( std::pair<T1, T2>* p ); | (3) | (library fundamentals TS)
template< class T1, class T2, class U, class V > void construct( std::pair<T1, T2>* p, U&& x, V&& y ); | (4) | (library fundamentals TS)
template< class T1, class T2, class U, class V > void construct( std::pair<T1, T2>* p, const std::pair<U, V>& xy ); | (5) | (library fundamentals TS)
template< class T1, class T2, class U, class V > void construct( std::pair<T1, T2>* p, std::pair<U, V>&& xy ); | (6) | (library fundamentals TS)

Constructs an object in allocated, but not initialized storage pointed to by p the provided constructor arguments. If the object is of type that itself uses allocators, or if it is std::pair, passes `this->resource()` down to the constructed object.

1) If std::uses_allocator<U, memory_resource*>::value == false (the type `U` does not use allocators) and std::is_constructible<U, Args...>::value == true, then constructs the object as if by ::new((void *) p) U(std::forward<Args>(args)...);.

Otherwise, if std::uses_allocator<U, memory_resource*>::value == true (the type `U` uses allocators, e.g. it is a container) and std::is_constructible<U, std::allocator_arg_t, memory_resource*, Args...>::value == true, then constructs the object as if by ::new((void *) p) U(std::allocator_arg, this->resource(), std::forward<Args>(args)...);.

Otherwise, if std::uses_allocator<U, memory_resource*>::value == true (the type `U` uses allocators, e.g. it is a container) and std::is_constructible<U, Args..., memory_resource*>::value == true, then constructs the object as if by ::new((void *) p) U(std::forward<Args>(args)..., this->resource());.

Otherwise, the program is ill-formed.

2) First, if either `T1` or `T2` is allocator-aware, modifies the tuples x and y to include `this->resource()`, resulting in the two new tuples `xprime` and `yprime`, according to the following three rules:

2a) if `T1` is not allocator-aware (std::uses_allocator<T1, memory_resource*>::value == false) and std::is_constructible<T1, Args1...>::value == true, then `xprime` is x, unmodified.

2b) if `T1` is allocator-aware (std::uses_allocator<T1, memory_resource*>::value == true), and its constructor takes an allocator tag (std::is_constructible<T1, std::allocator_arg_t, memory_resource*, Args1...>::value == true, then `xprime` is std::tuple_cat(std::make_tuple(std::allocator_arg, this->resource()), std::move(x)).

2c) if `T1` is allocator-aware (std::uses_allocator<T1, memory_resource*>::value == true), and its constructor takes the allocator as the last argument (std::is_constructible<T1, Args1..., memory_resource*>::value == true), then `xprime` is std::tuple_cat(std::move(x), std::make_tuple(this->resource())).

2d) Otherwise, the program is ill-formed.

Same rules apply to `T2` and the replacement of y with `yprime`.

Once `xprime` and `yprime` are constructed, constructs the pair p in allocated storage as if by ::new((void *) p) pair<T1, T2>(std::piecewise_construct, std::move(xprime), std::move(yprime));.

3) Equivalent to construct(p, std::piecewise_construct, std::tuple<>(), std::tuple<>()), that is, passes the memory resource on to the pair's member types if they accept them.

4) Equivalent to

construct(p, std::piecewise_construct, std::forward_as_tuple(std::forward<U>(x)),
std::forward_as_tuple(std::forward<V>(y)))

5) Equivalent to

construct(p, std::piecewise_construct, std::forward_as_tuple(xy.first),
std::forward_as_tuple(xy.second))

6) Equivalent to

construct(p, std::piecewise_construct, std::forward_as_tuple(std::forward<U>(xy.first)),
std::forward_as_tuple(std::forward<V>(xy.second)))

### Parameters

p | - | pointer to allocated, but not initialized storage
args... | - | the constructor arguments to pass to the constructor of `T`
x | - | the constructor arguments to pass to the constructor of `T1`
y | - | the constructor arguments to pass to the constructor of `T2`
xy | - | the pair whose two members are the constructor arguments for `T1` and `T2`

### Return value

(none)

### Notes

This function is called (through std::allocator_traits) by any allocator-aware object, such as std::vector, that was given a std::polymorphic_allocator as the allocator to use. Since `memory_resource*` implicitly converts to `polymorphic_allocator`, the memory resource pointer will propagate to any allocator-aware subobjects using polymorphic allocators.

### See also

construct [static] | constructs an object in the allocated storage (function template)
construct (until C++20) | constructs an object in allocated storage (public member function of `std::allocator<T>`)

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